[PECOFF] Parse .drectve section before reading other file contents.

Currently .drectve section contents are parsed after other sections are parsed.
That order may result in wrong results if other sections depend on command line
options in the directive section.

For example, if a weak symbol is defined using /alternatename option in the
directive section, we have to read it first and then read the text section
contents. Otherwise the weak symbol won't be defined.

This patch changes the order to fix the issue.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198071 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Change the DLL entry symbol.

I'm not 100% sure but it looks like DLL entry symbol (DLL initializer function
name) should be _DllMainCRTStartup@12. The reason why I'm not very sure is
because I have no idea what "@12" suffix is, but without it the symbol won't
be resolved...

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198072 91177308-0d34-0410-b5e6-96231b3b80d8